Methods and compositions for inhibiting polystyrene formation during styrene production

ABSTRACT

Methods and compositions are provided for inhibiting the polymerization of a vinyl aromatic monomer, such as styrene monomer, during elevated temperature processing or distillation thereof. The compositions comprise an inhibitor combination of (A) a hydroxylamine and (B) a stable free radical plus a retarder that is either; (C) dinitrobutylphenol or (D) quinone methide. The ratio of (A) to (B) ranges from about 5% (A) to 95% (B) to about 95% (A) to about 5% (B). The compositions are added to the vinyl aromatic monomer in amounts sufficient to prevent polymerization during the distillation process. Typically the inhibitor combination is added to the vinyl aromatic monomer in an amount ranging from about 10 to 150 ppm of the monomer. Retarders are typically added in an amount ranging from 50 to 1500 ppm of the vinyl aromatic monomer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of U.S. patent application Ser. No. 13/235,004 filed Sep. 16, 2011 which is hereby incorporated herein by reference.

FIELD OF INVENTION

The invention pertains to methods and compositions for inhibiting polymerization of a vinyl aromatic monomer, such as styrene monomer, during the purification or distillation step of the production process.

BACKGROUND OF THE INVENTION

Common industrial methods for producing styrene typically include separation and purification processes, such as distillation, to remove unwanted impurities. Unfortunately, purification processes carried out at elevated temperatures result in an increased rate of undesired polymerization. Heat polymerization is rapid. This polymerization is undesirable during many stages of the manufacturing and processing of styrene monomers, as it results not only in a loss of the desired monomer end-product, but a loss of production efficiency caused by polymer formation and/or agglomeration of polymer on process equipment.

To minimize these losses, distillation is generally carried out under vacuum. In addition, inhibitors and retarders are frequently used. Inhibitors include hydroxylamines and stable free radicals. True inhibitors have a termination rate that is greater than 10 times the polymerization propagation rate. These inhibitors completely eliminate polymer formation for a period of time, known as the induction period. Retarders include dinitrobutylphenol and quinone methides. Retarders are not true inhibitors as their systems have no induction period and the termination to propagation rate is less than 10. Instead, retarders keep polymerization at a low, controlled rate.

Hydroxylamines work well at temperatures between 110° C. to 125° C., and stable free radicals work well at temperatures below 110° C. However, styrene is purified at temperatures that vary from one manufacturing plant to another. Moreover, temperatures will also vary with each distillation column at the same manufacturing plant. Thus, temperature ranges will vary from about 75° C. to 125° C. during the typical styrene distillation process, creating strong need for methods that reduce polymerization across the 75° C. to 125° C. temperature range.

SUMMARY OF THE INVENTION

There are three exemplary embodiments of the invention. In the first embodiment, a method is provided for inhibiting the polymerization of a vinyl aromatic monomer such as styrene monomer. The method comprises adding an effective polymerization inhibiting amount of a combined treatment, or composition, to the monomer medium just before the distillation process. The combined treatment comprises (A) a hydroxylamine and (B) a stable free radical with a ratio ranging from about 5% (A) to 95% (B) to about 95% (A) to about 5% (B). From about 10-150 ppm of (A) and (B) collectively is brought into contact with the requisite vinyl aromatic monomer based on 1 million parts of the monomer.

In the second embodiment of the invention, a vinyl aromatic monomer anti-polymerization composition is added to the monomer medium just before the distillation process. This composition includes the combined treatment of (A) a hydroxylamine and (B) a stable free radical, plus (C) a dinitrobutylphenol retarder. From about 10-150 ppm of (A) and (B) collectively is brought into contact with the requisite vinyl aromatic monomer based on 1 million parts of the monomer. From about 50 to 1500 ppm of the dinitrobutylphenol retarder is brought into contact with the vinyl aromatic monomer based on 1 million parts of the monomer.

In the third embodiment of the invention, a vinyl aromatic monomer anti-polymerization composition is added to the monomer medium just before the distillation process. This composition includes the combined treatment of (A) a hydroxylamine and (B) a stable free radical, plus (D) a quinone methide retarder. From about 10-150 ppm of (A) and (B) collectively is brought into contact with the requisite vinyl aromatic monomer based on 1 million parts of the monomer. From about 50 to 1500 ppm of the quinone methide retarder is brought into contact with the vinyl aromatic monomer based on 1 million parts of the monomer.

The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, is not limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Range limitations may be combined and/or interchanged, and such ranges are identified and include all the sub-ranges stated herein unless context or language indicates otherwise. Other than in the operating examples or where otherwise indicated, all numbers or expressions referring to quantities of ingredients, reaction conditions and the like, used in the specification and the claims, are to be understood as modified in all instances by the term “about”.

“Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, or that the subsequently identified material may or may not be present, and that the description includes instances where the event or circumstance occurs or where the material is present, and instances where the event or circumstance does not occur or the material is not present.

As used herein, the terms “comprises”, “comprising”, “includes”, “including”, “has”, “having”, or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article or apparatus that comprises a list of elements is not necessarily limited to only those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

The singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.

The methods of the present invention can control fouling caused by the polymerization of monomer on processing equipment used in the manufacturing of styrene monomer. The instant invention may be used as a process inhibitor, alone or with a retarder, which is employed during preparation and processing (e.g., employing heat) of the styrene monomer.

The invention will now be described in conjunction with the following examples which should be viewed as being illustrative of the invention and should not be deemed to limit the invention in any manner.

In accordance with the invention, a combined treatment comprising (A) a hydroxylamine and (B) a stable free radical are conjointly utilized to inhibit polymerization of a vinyl aromatic monomer such as styrene.

Hydroxylamines have the functional group —NOH— and the general formula R₁—NOH—R₂. R₁ and R₂ may be the same or different and are hydrogen, alkyl, aryl, alkaryl, or hydroxyalkyl groups and preferably have three to about twenty carbon atoms. At present, the preferred compound (A) is 2-propanol, 1,1′-(hydroxyimino)bis, or 1,1′-(hydroxyimino)dipropan-2-ol, or bis 1,1-(2-Propanol)-hydroxyimine.

In addition to the hydroxylamine, the combined treatment has a stable free radical. Stable free radicals include nitroxyl compounds. Among these nitroxyl compounds, the preferred compound (B) for the combined treatment is 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy, or tetramethylpiperidino-N-oxyl, or 1-oxyl-2,2,6,6-tetramethyl-4-piperidinol.

In accordance with the invention, suitable retarders include dinitrobutylphenol (C) and quinone methide (D). The quinone methide (QM) compound has the formula:

wherein R₃ and R₄ are independently H, C₁ to C₁₈ alkyl, C₅ to C₁₂ cycloalkyl; or C₇ to C₁₅ phenylalkyl, and R₅ is aryl, or aryl substituted with C₁ to C₆ alkyl, alkoxy, hydroxy, nitro, amino, carboxy or mixtures thereof.

The compositions of the present invention are effective at inhibiting polymerization of vinyl aromatic monomers under processing conditions. These processing conditions include but are not limited to preparation, purification, distillation and vacuum distillation processes.

Styrene, for example, is typically processed at temperatures between 75° C. and 125° C. The compositions of the present invention are effective at inhibiting the polymerization of styrene over this range of temperatures.

The vinyl aromatic monomers that are treated by the compositions of the present invention include but are not limited to styrene, bromostyrene, divinylbenzene, and α-methylstyrene. The compositions of the present invention are particularly efficacious at inhibiting and/or retarding the polymerization of styrene monomer.

The total amount of hydroxylamine (A) and stable free radical (B) and dinitrobutylphenol (C) or quinone methide (D) used in the methods of the present invention is that amount which is sufficient to inhibit polymerization of vinyl aromatic monomers. This amount will vary according to the conditions under which the vinyl aromatic monomer is being processed, contaminants in the system, and the temperature of the system. At higher processing temperatures and higher monomer contamination, larger amounts of the inhibiting composition are required.

Accordingly, it is possible to create a more robust method of inhibiting vinyl aromatic monomer polymerization than is obtained by the use of either a hydroxylamine or stable free radical by itself. This method works well in both the low and high temperature zones of styrene distillation thus lowering the total amount of polymerization. For purposes of the present invention, the term “effective inhibiting amount” is that amount which is effective at inhibiting vinyl aromatic monomer polymerization.

The compositions of the present invention can be introduced into the vinyl aromatic monomer by any conventional method at any point of the processing system, either as separate and individual ingredients or as a combination of ingredients. Preferably, the compositions are added just before the styrene purification or distillation process.

The compositions of the present invention may be added to the vinyl aromatic monomer as a solution using a suitable liquid carrier or solvent. Any solvent that is compatible with the individual ingredients of the composition and the vinyl aromatic monomer to be treated may be employed. It is often desirable to dissolve the inhibitors in the monomer to which the inhibitor is being added to avoid introducing additional impurities in the monomer. Exemplary liquid carriers include water and non-polar organic solvents, such as ethyl benzene.

In accordance with the invention, both a hydroxylamine (A) and a stable free radical (B) are combined to inhibit polymerization of a vinyl aromatic monomer such as styrene. An effective polymerization inhibiting amount of a combined treatment, or composition, is added to the monomer medium just before the distillation process. The combined treatment comprises (A) to (B) with a ratio ranging from about 5% (A) to 95% (B) to about 95% (A) to about 5% (B). From about 10-150 ppm of (A) and (B) collectively is brought into contact with the requisite vinyl aromatic monomer based on 1 million parts of the monomer.

In another embodiment of the invention, a vinyl aromatic monomer anti-polymerization composition is added to the monomer medium just before the distillation process. This composition includes the combined treatment of (A) a hydroxylamine and (B) a stable free radical, plus (C) a dinitrobutylphenol retarder. From about 10-150 ppm of (A) and (B) collectively is brought into contact with the requisite vinyl aromatic monomer based on 1 million parts of the monomer. From about 50 to 1500 ppm of the dinitrobutylphenol retarder is brought into contact with the vinyl aromatic monomer based on 1 million parts of the monomer.

In a third embodiment of the invention, a vinyl aromatic monomer anti-polymerization composition is added to the monomer medium just before the distillation process. This composition includes the combined treatment of (A) a hydroxylamine and (B) a stable free radical, plus (D) a quinone methide retarder. From about 10-150 ppm of (A) and (B) collectively is brought into contact with the requisite vinyl aromatic monomer based on 1 million parts of the monomer. From about 50 to 1500 ppm of the quinone methide retarder is brought into contact with the vinyl aromatic monomer based on 1 million parts of the monomer.

While this invention has been described in conjunction with the specific embodiments described above, it is evident that many alternatives, combinations, modifications and variations are apparent to those skilled in the art. Accordingly, the preferred embodiments of this invention, as set forth above are intended to be illustrative only, and not in a limiting sense. Various changes can be made without departing from the spirit and scope of this invention. Therefore, the technical scope of the present invention encompasses not only those embodiments described above, but also all that fall within the scope of the appended claims.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated processes. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. These other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. 

What is claimed is:
 1. A method for inhibiting the polymerization of a vinyl aromatic monomer comprising: adding to the monomer an effective polymerization inhibitor combination comprising (A) a hydroxylamine; and (B) stable free radical.
 2. The method of claim 1, wherein the ratio of (A) to (B) ranges from about 5% (A) to 95% (B) to about 95% (A) to about 5% (B).
 3. The method of claim 2, wherein the hydroxylamine (A) is 2-propanol, 1,1′-(hydroxyimino)bis.
 4. The method of claim 3, wherein the stable free radical (B) is 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy.
 5. The method of claim 1, wherein the inhibitor combination is added to the vinyl aromatic monomer in an amount ranging from about 10 to about 150 ppm of the vinyl aromatic monomer.
 6. The method of claim 5, wherein a retarder is added to the vinyl aromatic monomer in an amount ranging from about 50 to about 1500 ppm of the vinyl aromatic monomer.
 7. The method of claim 6, wherein the retarder is (C) dinitrobutylphenol.
 8. The method of claim 6, wherein the retarder is (D) quinone methide.
 9. The method of claim 1, wherein the vinyl aromatic monomer is selected from the group consisting of styrene, bromostyrene, divinylbenzene and α-methylstyrene.
 10. The method of claim 1, wherein temperature of polymerization of the vinyl aromatic monomer ranges from about 75° C. to about 125° C.
 11. The method of claim 6, wherein the inhibitor combination and retarder further comprise a liquid carrier including water or a non-polar organic solvent.
 12. The method of claim 6, further comprising the step of heating the vinyl aromatic monomer.
 13. The method of claim 6, further comprising the step of distilling the vinyl aromatic monomer to remove impurities therefrom. 